The present invention relates to a method of controlling printed density in a sublimation type thermal transfer recording apparatus enabling recording of multiple gradations.
A thermal transfer recording apparatus is known from, for example, Japanese Utility Model Laid-Open Publication No. 62-85444 in which a platen roller and a thermal head having a plurality of heating resistors arranged in a line are brought into pressing contact with each other and a recording medium and an ink sheet are overlapped so as to be inserted in between the platen roller and the thermal head such that printing of multiple gradations is performed on the recording medium by heating desired ones of the heating resistors. In the case where printing of uniform density is performed on the recording medium by using the known thermal transfer recording apparatus, printed density on the recording medium in a main scanning direction extending in parallel with the line of the heating resistors on the thermal head becomes nonuniform.
FIGS. 1(a) to 1(f) show printed density nonuniformities at various printed densities, respectively in the known thermal transfer recording apparatus. For example, the indication "20th gr." of FIG. 1(a) represents printed density at the 20th gradation counted from the lightest gradation in case the known thermal transfer recording apparatus enables printing of 128 gradations. It will be understood from FIGS. 1(a) to 1(f) that printed density nonuniformities at the respective printed densities have a substantially similar shape in the main scanning direction but a sum of absolute values of differences between a reference printed density and actual printed densities varies greatly according to the printed densities.
It is considered that printed density nonuniformity is caused by dispersion in resistance values of the heating resistors on the thermal head, improper flatness of a glazed layer of each heating resistor, defective flatness of the platen roller which is disposed in pressing contact with the heating resistors such that the recording medium is interposed therebetween, etc.
In order to correct printed density nonuniformity caused by dispersion of resistance values of the heating resistors, a method is proposed in which the resistance values of the heating resistors are measured and data on errors between the measured resistance values and a predetermined resistance value is stored in a read-only memory (ROM) such that printed density is corrected by sequentially fetching the error data from the ROM at the time of printing.
However, it is time-consuming and expensive to measure the resistance values of the heating resistors and input the error data into the ROM. Meanwhile, since other causes of printed density nonuniformity are not removed, corrected printing does not achieve desired density.